Option to use file polling with cargo watch

[paul-hansen]

Cargo watch has a --poll flag that is useful in situations where you can't rely on filesystem events being consistent. This PR exposes that flag as a field on HotReloadPlugin.

This is especially useful when using WSL as you might be modifying the files in Windows and running in WSL or vice versa and WSL sadly doesn't send the file events between the two systems. I added some simple WSL detection to enable polling by default for those cases.

I also made a slight change to the feature cfg for build() to reduce some indentation that I added in my previous PR.

[DGriffin91]

@paul-hansen Thanks for the contribution! I'm assuming you tested this with WSL? I'm not currently setup to test it. The WSL check looks a bit janky, but if that's a somewhat accepted way of checking, I'm fine with it. Make sure to use cargo fmt if you're not already.

[paul-hansen]

I've been mostly using WSL for making Windows builds and running them while in a WSL path. (Rust compiles faster in WSL.) So I've tested the non-linux detection, but I haven't gotten Bevy wsl linux builds to actually run so I haven't tested the linux check. I borrowed most of that from: https://docs.rs/wsl/0.1.0/src/wsl/lib.rs.html#1-27

This is the best source for confirmation for how to detect it I've found: microsoft/WSL#423 (comment) (comment from a wsl developer).

Here's the output of /proc/sys/kernel/osrelease from my WSL instance:

Make sure to use cargo fmt if you're not already.

I am, clippy too. Not sure why it wasn't formatting what I fixed with the "Fix indentation" commit, I thought that was kind of odd. Something tripping it up.

[paul-hansen]

If you want I can remove the WSL check and just have this PR add the poll option and default to false. That way you don't have to worry about maintaining it.

[DGriffin91]

Your sources seem like reason enough for me. If there's issues in the future we'll address them then.